1. Field of the Invention
The present invention generally relates to methods and apparatus for stimulating certain areas of the brain to treat epilepsy by modulation of electrical activity of neural tissue in selected areas of the brain.
2. Description of Related Art
Epilepsy, or seizure disorder, is a chronic neurological condition characterized by recurrent seizures that begin in the brain and are associated with excessive or abnormal synchronization of neural activity. It has been estimated that about 1% of the population suffer from some form of epilepsy. The type of seizure experienced by an epilepsy sufferer during an epileptic event varies from one individual to another, but is usually one of the following types: absence seizures, partial seizures (petit mal), complex partial seizures, generalized tonic-clonic contractions of muscles (grand mal), temporal lobe seizures and generalized motor seizures. Partial onset seizures begin in a single part of the brain and remain localized to only that area (focal), while general onset seizures arise throughout the entire brain simultaneously (multi-foci). In some instances, a partial onset seizure can progress to involve much of the brain, in which case the seizure is said to be “secondarily generalized.” Some seizures result in a loss of conscious awareness by the individual, and are termed “complex” seizures. Although consciousness is unimpaired in an individual suffering from “simple” seizures, that person may experience other symptoms such as sensory distortions, involuntary movements, and loss of muscle tone. The behavioral aspects of a seizure often reflect a function of the area or areas of the brain where the abnormal electrical activity takes place.
Neurologists recognize a number of distinct epilepsy syndromes which are usually classified according to the area or areas of the brain associated with the seizures. Among these are frontal lobe epilepsy, occipital lobe epilepsy, mesial temporal lobe epilepsy, parietal lobe epilepsy, benign myoclonic epilepsy in infants, juvenile myoclonic epilepsy, childhood absence epilepsy, juvenile absence epilepsy, epilepsy with generalized tonic clonic seizures in childhood, infantile spasms (West syndrome), Lennox-Gastaut syndrome, progressive myoclonus epilepsies, febrile fits, epilepsy with continuous spike and waves in slow wave sleep (ESES), Laudau Kleffner syndrome, and Rasmussen's syndrome.
The reasons why susceptible individuals develop epilepsy are not completely understood and there are many possible causes. Some epilepsy sufferers require treatment for many years to keep seizures under control. Treatment of epilepsy typically includes medication, and sometimes neurosurgical intervention (lesioning). U.S. Pat. No. 6,923,784 (Medtronic) describes drug infusion into the brain. Electrical stimulation of the vagus nerve using an implantable stimulator has proven successful in reducing the severity and/or frequency of seizures in some cases. U.S. Pat. No. 5,928,272 (Cyberonics, Inc.) describes such a device that is activated to generate a therapeutic waveform upon detecting a time rate of change in the patient's heart rate which is sufficiently abrupt and of sufficient magnitude to be inconsistent with normal physical activity, indicative of an imminent epileptic seizure. U.S. Pat. No. 6,920,357 (Osorio) describes certain vagal nerve stimulation techniques for treatment of epileptic seizures. U.S. Pat. No. 6,339,725 (Southern Illinois University) describes certain methods of modulating aspects of brain neural plasticity by vagus nerve stimulation. It is said that vagus nerve stimulation has been shown to cause activation of several parts of the brain that are specifically involved in cognitive processing, memory, learning, sensory and motor processing, and affects regions of the brain that are prone to developing epilepsy or which regulate the development of epilepsy. Studies have reportedly demonstrated that vagus nerve stimulation activates the amygdala and cingulate cortex, which are involved in learning and cognitive processing. Such stimulation also activates several thalamic nuclei which serve relay functions. In addition, it activates several sensory nuclei, including the auditory, visual, and somatic sensory systems. Finally, vagus nerve stimulation reportedly activates monoaminergic nuclei, especially the locus ceruleus and A5 groups, which provide norepinephrine to the brain. Another important site of activation is the dorsal raphe which provides serotonin to the brain.
There have also been reports of attempts to treat epilepsy by direct electrical stimulation of the brain (DBS). In DBS, neurons in the immediate vicinity of the electrodes are induced to fire (i.e., are recruited) by the electrical stimulus to modify the electrical activity in that area of the brain. U.S. Pat. No. 6,016,449 describes a brain stimulator implanted in the cranium and having leads terminating with electrodes in contact with brain tissue. U.S. Pat. No. 6,944,501 (Neurospace, Inc.) describes a neurostimulator that is configured to treat epilepsy and other neurological disorders using certain stimulation strategies including changing various pulse parameters, during the imposition of a burst of pulses. U.S. Patent Application Publication No. 2005/0021104 (DiLorenzo) describes a neurological control system for generating neural modulation signals delivered to a nervous system component through one or more intracranial stimulating electrodes in accordance with certain treatment parameters. Such treatment parameters may be derived from a neural response to previously delivered neural modulation signals sensed by sensors configured to sense a particular characteristic indicative of a neurological or psychiatric condition. U.S. Patent Application Publication No. 2005/0182453 (Advanced Bionics Corporation) states that because vagal nerve stimulation has been demonstrated to have anticonvulsant effects, it is said to be likely that changes in synaptic transmission in the nucleus of the solitary tract, a primary site at which vagal afferents terminate, can regulate seizure susceptibility. Certain drawbacks of available DBS implantable systems are described, and a method of treating epilepsy by implanting an electrical control unit within the patient to apply a stimulus including a current having a frequency of 400 Hertz or greater is described in U.S. Patent Application Publication No. 2005-0182453. It is said that the stimulus may additionally or alternatively include infusion of one or more drugs into the stimulation site. Stimulation sites in the brain are said to include seizure foci such as the thalamus (including centromedian, anterior, and ventrolateral nuclei and any other site of thalamic relay neurons), hippocampus, amygdala, cerebellum, nucleus tractus solitarius (NTS), locus coeruleus, and mesial temporal lobe. The stimulation site may also include any nerve branching from one of those sites, or an area of the brain that may propagate a seizure or that demonstrates increased activity in epileptics relative to non-epileptic controls.
Alternative ways to treat patients suffering from an epilepsy syndrome that is not sufficiently responsive to conventional therapies are needed.